Inputs

Outputs

Description

CLUSTER instructs PostgreSQL to cluster the table specified by
table approximately based on the
index specified by indexname. The
index must already have been defined on tablename.

When a table is clustered, it is physically reordered based on
the index information. The clustering is static. In other words,
as the table is updated, the changes are not clustered. No
attempt is made to keep new instances or updated tuples
clustered. If one wishes, one can re-cluster manually by issuing
the command again.

Notes

The table is actually copied to a temporary table in index
order, then renamed back to the original name. For this reason,
all grant permissions and other indexes are lost when
clustering is performed.

In cases where you are accessing single rows randomly within
a table, the actual order of the data in the heap table is
unimportant. However, if you tend to access some data more than
others, and there is an index that groups them together, you
will benefit from using CLUSTER.

Another place where CLUSTER is
helpful is in cases where you use an index to pull out several
rows from a table. If you are requesting a range of indexed
values from a table, or a single indexed value that has
multiple rows that match, CLUSTER will
help because once the index identifies the heap page for the
first row that matches, all other rows that match are probably
already on the same heap page, saving disk accesses and
speeding up the query.

There are two ways to cluster data. The first is with the
CLUSTER command, which reorders the
original table with the ordering of the index you specify. This
can be slow on large tables because the rows are fetched from
the heap in index order, and if the heap table is unordered,
the entries are on random pages, so there is one disk page
retrieved for every row moved. PostgreSQL has a cache, but the majority
of a big table will not fit in the cache.

Another way to cluster data is to use

SELECT columnlist INTO TABLE newtable
FROM table ORDER BY columnlist

which uses the PostgreSQL
sorting code in the ORDER BY clause to match the index, and
which is much faster for unordered data. You then drop the old
table, use ALTER TABLE...RENAME to
rename newtable to the old
name, and recreate the table's indexes. The only problem is
that OIDs will not be
preserved. From then on, CLUSTER should
be fast because most of the heap data has already been ordered,
and the existing index is used.